The demands for radio frequency spectrum and increased channel throughput have forced satellite designers to operate over increasingly band limited channels. Bandwidth efficient modulation is now a necessity and requires that more attention be paid to link distortion effects. This becomes particularly important when Traveling Wave Tube Amplifiers (TWTAs) are operated at or near saturation on the spacecraft. The present invention is directed to method and apparatus for measuring such distortions, and monitor on-orbit Ka band transponder performance.
Transponder distortions associated with the use of TWTAs can be separated into two categories: linear effects and non-linear effects. Linear effects include such distortions as tilt and ripple across the TWTA operating bandwidth. Non-linear effects include such distortions as Amplitude to Amplitude Conversion (AM/AM) and Amplitude to Phase conversion (AM/PM), which occur when fluctuations in the modulated signal envelope are present. These fluctuations are directly related to the degree of channel band limiting present on the link and the modulation format. Both types of distortion affect the overall link performance. It is therefore desirable to obtain an accurate characterization of the transponder""s behavior, particularly at saturation.
Conventional approaches for satellite transponder characterization focus on the pre-flight measurement of individual subsystem distortion parameters and use such measurements in a xe2x80x9csystemxe2x80x9d model to predict end-to-end link performance. Such techniques typically involve considerable investments in test equipment and time, and require elaborate test procedures. Most of these tests typically do not employ actual wideband signals, nor do they necessarily treat the satellite transponder as a whole. Typically, a xe2x80x9cnarrowbandxe2x80x9d swept tone response is used to measure many of the desired parameters. Due to the inherent non-linearity of TWTAs, however, such techniques will not produce an accurate characterization at saturation. The transponder characterization, and the model which arises, may not fully reflect the composite interactions of transponder distortions on key performance parameters, e.g. Bit Error Rate (BER). In this sense, measurement techniques which employ wideband test signals, and are capable of treating the transponder as a single unit, are highly desirable.
The requirement to have access to internal inputs and outputs when utilizing conventional approaches also limits the degree of resting which can be performed at the system level (integrated transponder) to measure distortion parameters. Conventional system level tests are therefore primarily limited to functional capability and overall BER performance.
Over the last few years, several efforts of applicant""s assignee have resulted in the development of a cost effective hardware test set capable of characterizing non-linear transponders. The Advanced Channel Characterization (ACC) system is capable of measuring, among other parameters, the effective linearized frequency response of a saturated transponder. The technique can be used with engineering models, brass boards, and flight hardware in the laboratory, and can also be used on-orbit. The approach uses a modulated data sequence representative of actual data, therefore occupying the entire transponder bandwidth. The technique has been successfully validated in hardware tests at both Ku and Ka band, and at bandwidths up to 1 GHz. The present invention concentrates on the frequency response measurement capabilities of the ACC device.
The object of the invention is to provide method and apparatus for performance characterization of satellite transponders, and in general the characterization of communication system elements and RF networks exhibiting either linear or non-linear transfer functions.
Another object of the invention is to provide method and apparatus for performance characterization of Ka band transponders.